Adds a skeleton component app for Always-On Hotwording opt-in flow.

chrome/browser/search/hotword_service.h

Index: chrome/browser/search/hotword_service.h
diff --git a/chrome/browser/search/hotword_service.h b/chrome/browser/search/hotword_service.h
index 457319ed27377b68af88ada2582b1c63b5740b6c..f69c9d8cd804d7706284a6a08c3fdf590fb43202 100644
--- a/chrome/browser/search/hotword_service.h
+++ b/chrome/browser/search/hotword_service.h
@@ -73,6 +73,10 @@ class HotwordService : public content::NotificationObserver,
   void EnableHotwordExtension(ExtensionService* extension_service);
   void DisableHotwordExtension(ExtensionService* extension_service);
 
+  // Handles launching the Hotword Audio Verification extension when the user
+  // turns it on via the settings menu.
+  void OnHotwordAlwaysOnSearchEnabledChanged(const std::string& pref_name);
+
   // Handles enabling/disabling the hotword extension when the user
   // turns it off via the settings menu.
   void OnHotwordSearchEnabledChanged(const std::string& pref_name);